The present invention relates to an ultrasonic motor which has a driving circuit that utilizes a piezoelectric element.
The ultrasonic motor has an oscillating voltage applied to its piezoelectric element to induce a traveling wave or a standing wave on the surface of a resilient member and thereby drive a moving member which is placed in pressure contact with the resilient member. The frequency of the applied oscillating voltage signal must be equal to or close to a resonance frequency f.sub.r of the ultrasonic motor.
In a conventional type ultrasonic motor, the measurement of the resonance frequency f.sub.r is made beforehand and the drive signal is applied to the ultrasonic motor by means of a reference oscillator.
The conventional method of taking measurement of f.sub.r in advance and using a reference oscillator in producing the drive signal has a problem that forming an oscillator tuned to f.sub.r is not efficient in terms of mass-productivity as there are variations in electrical elements. Secondly, if the drive frequency is fixed by the reference oscillator, any change in f.sub.r and current consumption--which may result from variations in energy loss or heat in the ultrasonic motor itself and from variations in ambient temperatures, contact pressure of the moving member and applied signal amplitude--will bring the ultrasonic motor to a halt.
An ultrasonic motor to stably drive the ultrasonic motor has been filed by this applicant in Japan.
This type of ultrasonic motor makes use of the fact that the current flowing through the piezoelectric element becomes the maximum at the resonance frequency f.sub.r of the motor. That is, a resistor is used to convert the current into a voltage signal, which is rectified and amplified and then fed back as a control voltage to a voltage-controlled oscillator (VCO). A reference voltage for the rectification and amplification is set such that an output frequency of the VCO settles at a frequency close to f.sub.r. This method is capable of making the output frequency of VCO follow changes in f.sub.r and current that would result from variations of ambient temperature and applied signal amplitude. The apparatus therefore is suited for a stationary type ultrasonic motor having small load variations. However, when the moving member's contact pressure changes (due, for example, to impacts from falling when the ultrasonic motor is employed in a wristwatch,) causing a sharp change in f.sub.r and current, the VCO output frequency cannot follow such sudden changes, so that there may be cases where the VCO output frequency becomes stabilized at a value deviated from f.sub. r, leaving the ultrasonic motor halted. Moreover, since the magnitude of f.sub.r and current changes due to variations in ambient temperature and applied signal amplitude varies from one motor to another, an additional process is required to adjust the follow-up characteristic of individual VCOs.